Annotation Type Nullable

A common Spring annotation to declare that annotated elements can be null under
some circumstance. Leverages JSR 305 meta-annotations to indicate nullability in Java
to common tools with JSR 305 support and used by Kotlin to infer nullability of Spring API.

Should be used at parameter, return value, and field level. Methods override should
repeat parent @Nullable annotations unless they behave differently.

Can be used in association with @NonNullApi or @NonNullFields to
override the default non-nullable semantic to nullable.